1. Field of the Invention
The subject invention relates to RF (radio frequency) systems and more particularly to an RF architecture for a dual band cellular telephone.
2. Description of Related Art
Mobile phones have recently gained widespread use throughout the world. Mobile phones communicate with a base station serving a predetermined area or cell of a cellular network system, such as GSM. Each base station has a limited bandwidth within which to operate, depending upon the particular transmission technique utilized by the base station. This limited bandwidth is separated into a plurality of channels, which are frequency-spaced evenly from one another, and these plurality of channels are used by the mobile phones within that base station's transmission area. As a result, each base station can only handle a limited number of phones. The number of possible phones is equal to the number of channels and time-slots on those channels available at the base station. Therefore, the frequency spacing between channels is minimized in order to maximize the number of channels supported by the operating bandwidth of the base stations.
The capacity of base stations in highly populated areas can become saturated during time periods of high use. Mobile phones currently operate as single band phones, where the transmitted signal frequency is within the bandwidth of a base station operating on the same transmission method as the mobile phone. Thus, there is a need for mobile phones to operate with dual band transmissions to increase system capacity, so that the system could select between two transmission frequency bands depending upon which bandwidth is less saturated and could provide a better signal quality.
Typically, in a conventional single band cellular phone, data to be transmitted by the telephone handset is fed to a transmitter including a differential encoder, where an in-phase component (I) and a quadrature component (Q) of the signal to be transmitted are created. The I and Q components are then passed through digital filters which give the modulation a particular shape. The resultant I and Q filtered signals are then modulated at a radio frequency for transmission and combined as a phase modulated signal. The phase modulated signal is then amplified to bring the signal to a desired power level for transmission. Digital modulators, such as a Gaussian minimum shift keying (GMSK) modulator, are typically used in digital wireless phones.
Most mobile phones are designed to be lightweight and portable, so that they may be easily carried on the person using the mobile phone, such as in their pocket or purse. It is therefore critical to design a mobile phone to be as small as possible, thus requiring the number of components to be minimized. With respect to design of a dual band mobile phone, these considerations present serious design problems, for example, because the use of two entirely separate transmitter and receiver circuits for the respective bands would result in a prohibitively large and complex phone. Additional problems confront implementation of a dual band phone arising from the potential for generation of unwanted frequency components.